The containment for a nuclear reactor is defined as the enclosure that provides environmental isolation to the nuclear steam supply system (NSSS) of the plant in which nuclear fission is harnessed to produce pressurized steam. A commercial nuclear reactor is required to be enclosed in a pressure retaining structure which can withstand the temperature and pressure resulting from the most severe accident that can be postulated for the facility. The most severe energy release accidents that can be postulated for a reactor and its containment can be of two types.
First, an event that follows a loss-of-coolant accident (LOCA) and involve a rapid large release of thermal energy from the plant's nuclear steam supply system (NSSS) due to a sudden release of reactor's coolant in the containment space. The reactor coolant, suddenly depressurized, would violently flash resulting in a rapid rise of pressure and temperature in the containment space. The in-containment space is rendered into a mixture of air and steam. LOCAs can be credibly postulated by assuming a sudden failure in a pipe carrying the reactor coolant.
Another second thermal event of potential risk to the integrity of the containment is the scenario wherein all heat rejection paths from the plant's nuclear steam supply system (NSSS) are lost, forcing the reactor into a “scram.” A station black-out is such an event. The decay heat generated in the reactor must be removed to protect it from an uncontrolled pressure rise.
More recently, the containment structure has also been called upon by the regulators to withstand the impact from a crashing aircraft. Containment structures have typically been built as massive reinforced concrete domes to withstand the internal pressure from LOCA. Although its thick concrete wall could be capable of withstanding an aircraft impact, it is also unfortunately a good insulator of heat, requiring pumped heat rejection systems (employ heat exchangers and pumps) to reject its unwanted heat to the external environment (to minimize the pressure rise or to remove decay heat). Such heat rejection systems, however, rely on a robust power source (off-site or local diesel generator, for example) to power the pumps. The station black out at Fukushima in the wake of the tsunami is a sobering reminder of the folly of relying on pumps. The above weaknesses in the state-of-the-art call for an improved nuclear reactor containment system.
Besides the foregoing containment cooling issues, a nuclear reactor continues to produce a substantial quantity of heat energy after it has been shut down. FIG. 20 shows a typical heat generation curve of a light water reactor subsequent to a scram (i.e., a sudden cessation of chain reaction by a rapid insertion of control rods or other means). In the current reactor designs, as noted above, the reactor's decay heat is removed by the plant's residual heat removal (RHR) system which utilizes a system of pumps and heat exchangers to convey the heat energy to a suitable source of cooling water maintained by the plant. As can be seen from FIG. 20, the reactor's decay heat begins to attenuate exponentially with time but is still quite significant to threaten the reactor's safety if the generated heat were not removed (as was the case at Fukushima where the pumps needed to extract the reactors' heat failed because of submergence of their electric motors in the tsunami driven water surge). The Fukushima disaster provided a stark lesson in the vulnerability of forced flow (pump dependent) systems under extreme environmental conditions.
An improved reactor cooling system is desired.